The present invention is in a general way concerned with mobile radiocommunication systems.
The present invention is more particularly concerned with power control techniques used in such systems to improve performances (in terms of quality of service, of capacity, . . . etc.) despite the movements of users, i.e. despite continuous changes in their respective locations with respect to fixed infrastructures in such systems.
The present invention is in particular applicable to mobile radiocommunication systems of CDMA (“Code Division Multiple Access”) type. CDMA is a multiple access technique which makes it possible for several users to be simultaneously active on a same frequency, using different spreading codes.
As is known, CDMA systems use two types of power control techniques, a so-called open-loop power control technique, and a so-called closed loop power control technique (also called hereinafter CLPC). These power control techniques may be recalled for example for the uplink transmission direction, i.e. from MS (“Mobile Station”) to BTS (“Base Transceiver Station”). In the open-loop power control, a MS transmit power is controlled based on the power received by this MS from a BTS. In the CLPC, a MS transmit power is controlled based on the transmission quality of the link between this MS and a BTS, as estimated at this BTS.
The transmission quality of a link between a MS and a BTS depends on the ratio of the received signal power and the interference power, also called SIR (Signal-to-Interference Ratio). When the SIR of a MS is low, or equivalently when the powers of the other MSs are much higher than its power, its performances dramatically decrease. The CLPC algorithm enables to keep the SIR of each user as constant as possible.
The principle of the CLPC algorithm is that the BTS periodically estimates the SIR of the received signal from each MS, and compares this estimated SIR to a target SIR (SIRtarget). If the estimated SIR is lower than the target SIR, the BTS sends a command to the MS for the MS to increase its transmit power. Otherwise, the BTS sends a command to the MS for the MS to decrease its transmit power. The target SIR is chosen by the BTS as a function of the required quality of service.
Such a principle however requires that the environment is not changing too fast, in particular that MSs speed is not too high.
WO 98/51026 teaches to adapt the power control step size of the CLPC algorithm, in particular as a function of MSs mobility requirements (the BTS in particular assigning a larger power control step size for MSs having higher mobility requirements).
This however does not solve the problem that in fast changing environments, such as when MSs speed is high, the CLPC algorithm cannot track the SIR variations, which results in degraded performances. Indeed in this case the speed of SIR variations is much higher than the repetition period of the algorithm, which may result in sending to a MS at a given instant ti, a power control command obtained from environment requirements at instant ti.1, and which no longer corresponds to environment requirements at instant ti. This problem could be solved by reducing the repetition period of the algorithm, but this would result in a signalling increase between BTS and MS, and therefore in a non efficient use of available radio resources.
Therefore there is a general need to adapt such power control techniques, in particular to the case of fast changing environments, avoiding such drawbacks. The present invention meets such a purpose.